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www.atmos-chem-phys.net/15/7471/2015/ doi:10.5194/acp-15-7471-2015

© Author(s) 2015. CC Attribution 3.0 License.

Modulation of Saharan dust export by the North African dipole

S. Rodríguez1, E. Cuevas1, J. M. Prospero2, A. Alastuey3, X. Querol3, J. López-Solano1, M. I. García1,4, and S. Alonso-Pérez1,3,5

1Izaña Atmospheric Research Centre, AEMET, Santa Cruz de Tenerife, Spain

2Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, USA 3Institute of Environmental Assessment and Water Research, CSIC, Barcelona, Spain

4Department of Chemistry (T.U. Analytical Chemistry), Faculty of Science, University of La Laguna, Tenerife, Spain 5European University of the Canaries, Laureate International Universities, La Orotava, Tenerife, Spain

Correspondence to: S. Rodríguez (srodriguezg@aemet.es)

Received: 16 September 2014 – Published in Atmos. Chem. Phys. Discuss.: 24 October 2014 Revised: 24 April 2015 – Accepted: 23 June 2015 – Published: 10 July 2015

Abstract. We have studied the relationship between the long-term interannual variability in large-scale meteorology in western North Africa – the largest and most active dust source worldwide – and Saharan dust export in summer, when enhanced dust mobilization in the hyper-arid Sahara results in maximum dust impacts throughout the North At-lantic. We address this issue by analyzing 28 years (1987– 2014) of summer averaged dust concentrations at the high-altitude Izaña observatory (∼2400 m a.s.l.) on Tenerife, and satellite and meteorological reanalysis data. The summer me-teorological scenario in North Africa (aloft 850 hPa) is char-acterized by a high over the the subtropical Sahara and a low over the tropics linked to the monsoon. We measured the variability of this high–low dipole-like pattern in terms of the North African dipole intensity (NAFDI): the difference of geopotential height anomalies averaged over the subtrop-ics (30–32◦N, Morocco) and the tropics (10–13◦N, Bamako region) close to the Atlantic coast (at 5–8◦W). We focused on the 700 hPa standard level due to dust export off the coast of North Africa tending to occur between 1 and 5 km a.s.l. Vari-ability in the NAFDI is associated with displacements of the North African anticyclone over the Sahara and this has impli-cations for wind and dust export. The correlations we found between the 1987–2014 summer mean of NAFDI with dust at Izaña, satellite dust observations and meteorological re-analysis data indicate that increases in the NAFDI (i) result in higher wind speeds at the north of the Inter-Tropical Con-vergence Zone that are associated with enhanced dust export over the subtropical North Atlantic, (ii) influence the long-term variability of the size distribution of exported dust

par-ticles (increasing the load of coarse dust) and (iii) are asso-ciated with enhanced rains in the tropical and northern shifts of the tropical rain band that may affect the southern Sahel. Interannual variability in NAFDI is also connected to spa-tial distribution of dust over the North Atlantic; high NAFDI summers are associated with major dust export (linked to winds) in the subtropics and minor dust loads in the trop-ics (linked to higher rainfall), and vice versa. The evolution of the summer NAFDI values since 1950 to the present day shows connections to climatic variability (through the Sa-helian drought, ENSO (El Niño–Southern Oscillation) and winds) that have implications for dust export paths. Efforts to anticipate how dust export may evolve in future decades will require a better understanding of how the large-scale meteo-rological systems represented by the NAFD will evolve.

1 Introduction

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Díaz et al., 2012). Consequently, there is considerable in-terest in climate variability, the global distribution of dust (Adams et al., 2012; Ginoux et al., 2012) and dust micro-physical properties including particle size, which modulates dust impacts (Mahowald et al., 2014), e.g., the interaction with radiation (Otto et al., 2007), iron solubility and sup-ply to the ocean (Baker and Jickells, 2006), its role as cloud and ice nuclei (Welti et al., 2009), and health effects due to dust exposure (Pérez et al., 2008, 2014; Mallone et al., 2011; Díaz et al., 2012). During atmospheric transport, dust is removed by precipitation and by dry deposition, the lat-ter a process that is strongly size dependent. Dust size vari-ability is observed over timescales of individual dust events (∼days) (Ryder et al., 2013) and in ice cores, over thousands of years, linked to changes in wind speeds, transport path-ways and dust sources attributed to climate variability (Del-monte et al., 2004).

North Africa is the largest and most active dust source in the world (Ginoux et al., 2004, 2012; Huneeus et al., 2011). Dust mobilization experiences a marked seasonality. In win-ter, sources located in the southern Sahara and the Sahel (<20◦N) are especially active, linked to northeasterly dry (Harmattan – trade) winds that prompt dust export across the North African tropical coast (<15◦N) (Engelstaedter and Washington, 2007; Haywood et al., 2008; Menut et al., 2009; Marticorena et al., 2010). In summer, the northeasterly trade winds and the Inter-Tropical Convergence Zone (ITCZ) shift northward, enhancing emissions from Saharan sources and increasing dust export at subtropical latitudes (20–30◦N); concurrently the northward shift in the monsoon rain band to the southern Sahel tends to decrease Sahelian dust emis-sions (Engelstaedter and Washington, 2007; Knippertz and Todd, 2010; Ashpole and Washington, 2013, and references therein).

There is a major scientific interest in understanding the links between long-term variability in North African dust ex-port and climate. Dust sources in part of the Sahel have a hydrological nature (Ginoux et al., 2012); their emissions are affected by the summer variability in rainfalls and also by the North Atlantic Oscillation in winter, and this has had consequences for dust impacts on the tropical North At-lantic detected during, at least, four decades (Prospero and Lamb, 2003; Chiapello et al., 2005). In addition, the in-crease in commercial agriculture over the last two centuries coupled with droughts has had an impact on Sahelian dust emissions (Mulitza et al., 2010). In contrast, the Sahara is a hyper-arid environment (<200 mm yr−1) where natural non-hydrological dust sources (i.e., not associated with annual hydrological cycles) prevail (Ginoux et al., 2012), and dust emission variability is mainly controlled by winds (Engel-staedter and Washington, 2007; Ridley et al., 2014). A con-ceptual model explaining interannual variability in Saharan dust export has been proposed for the winter (e.g., North At-lantic Oscillation by Ginoux et al., 2004, and Chiapello et al., 2005), but not for summertime, when the highest dust

emis-sions occur in North Africa due to the enhanced activation of the subtropical Saharan sources (Prospero and Lamb, 2003; Ginoux et al., 2004; Chiapello et al., 2005; Tanaka and Chiba, 2006; Engelstaedter and Washington, 2007; Mulitza et al., 2010; Knippertz and Todd, 2012; Ridley et al., 2014). Do-herty et al. (2008) found that the trans-Atlantic dust transport of North African dust to the Caribbean is influenced by dis-placements in the Azores and Hawaiian anticyclones. In this study we have focused on the links between North African meteorology and dust export.

Starting in 1987 we have measured aerosols at the Izaña Global Atmospheric Watch (GAW) World Meteorological Organization (WMO) high-mountain observatory (28◦180N, 16◦290E; 2367 m a.s.l.) on Tenerife, which frequently lies under the main path of the high-altitude Saharan dust out-breaks. At night, when mountain upslope winds cease, Izaña is within the free troposphere airflows, frequently within the dust-laden Saharan air layer (SAL), which in summer is typi-cally located at altitudes between≈1 and 5 km a.s.l.(Adams et al., 2012; Nicholson, 2013; Tsamalis et al., 2013). Here we report on long-term measurements of summertime con-centrations of total dust (dustT) (1987–2014) and of dust par-ticles<2.5 µm (dust2.5) (2002–2014). Our 28-year observa-tion shows that there is a significant interannual variability in Saharan dust export in summer. Our research focuses on one key question: “What is the relationship between long-term interannual variability in Saharan dust export in summer and large-scale meteorology in North Africa?” To address this is-sue, we also used (i) the UV Aerosol Index determined by the Total Ozone Mapping Spectrometer and Ozone Monitor In-strument satellite-borne spectrometers (Herman et al., 1997) for studying long-term and interannual spatial distribution of dust and (ii) gridded meteorological National Center for En-vironmental Prediction/National Center for Atmospheric Re-search (NCEP/NCAR) re-analysis data (Kalnay et al., 1996) for studying the variability of large-scale meteorological pro-cesses.

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2 Methods

2.1 In situ dust measurements

We used in situ dust concentration data recorded between 1987 and 2014 at Izaña observatory. Here we present a brief description of the methods; details are included in Sect. S1 of the Supplement.

Dust concentrations were obtained by chemical analysis of aerosol samples collected on filters at the flow rate of 30 m3h−1. Throughout the almost three decades of observa-tions, several analytical methods have been used for deter-mining soluble species (SO=4, NO−3, and NH+4 by ion chro-matography and colorimetry), organic and elemental carbon (by TOT), elemental composition (INAA, AES and IPC-MS) and the content of dust (by the “weight of the ash residue after 14 h heating at 500◦C” method and by using the ele-mental composition data) in the aerosol samples; details of these methods and their use throughout the measurement pe-riod are included in Table S1 of the Supplement. In order to facilitate data comparison with other studies, dust concentra-tions are reported to mean pressure at sea level (1013 hPa) and normalized in such a way that aluminum accounts for 8 % dust (mean content of Al in soils). Here we report on dust concentrations in two size fractions: concentrations of total dust (dustT) from 1987 to 2014 and of dust particles with an aerodynamic diameter≤2.5 µm (dust2.5) from 2002 to 2014 (Rodríguez et al., 2012).

Dust concentrations were also calculated with a secondary complementary method based on number size distribution measurements (0.5 to 20 µm) performed with an optical par-ticle counter and an aerodynamic parpar-ticle sizer. These data were used for determining the aerosol volume tions and converting them to bulk aerosol mass concentra-tions using standard methods (Rodríguez et al., 2012). The good agreement (high linearity and low mean bias, 3–8 %) between these two methods (based on chemical analysis and on size distributions) is due to the very low aerosol vol-ume concentrations in the free troposphere during no dust events (typically<1 to<3 µg m−3; Rodríguez et al., 2009) and to the fact that the aerosol volume concentrations during dust events are by far dominated by dust, as evidenced by the chemical analysis (Rodríguez et al., 2011) and the ochre color of the aerosol samples (Fig. 1b).

These two dust databases (based on chemical and size dis-tribution methods) were used to assess the consistency of the observed year-to-year variability of dust. During the whole measurement period (25 July 1987–31 December 2014, ex-cluding the non-measurement period 11 October 1999–13 February 2002), dust concentration records are available for 8001 days, which leads to a data availability of 87.3 %. This record of aerosol dust concentration is among the longest in the world (after Barbados, started in 1965, Miami, 1972, and American Samoa, 1983) and probably the longest in the free

Figure 1. Saharan dust observations in Izaña. (a) Frequency of dust events (>10 µg m−3) in Izaña in the period 1987–2014. (b) Batch of filters with aerosol samples collected at Izaña for illustrating their typical ochre color due to dust.

troposphere and in several size fractions downwind of a dust large source (Rodríguez et al., 2012).

2.2 Satellite dust observations

We used UV Aerosol Index (AI) data from the Total Ozone Mapping Spectrometer – TOMS – (1979–2001) and from the Ozone Monitor Instrument – OMI – (2005–2014) spectrome-ters onboard satellites Nimbus 7 (TOMS 1979–1993), Earth Probe (TOMS 1996–2001) and Aura (OMI 2005–2014) to study the spatial and temporal variability of dust. Because of the UV absorption by some minerals (e.g., hematite, goethite), AI has been widely used in dust studies. This is a semi-quantitative parameter; AI values>1 are considered representative of an important dust load and the frequency of daily AI values>1 has been used for dust climatology (Prospero et al., 2002). In North Africa, the AI signal at the north of the summer tropical rain band is due to dust, whereas biomass burning aerosols transported from southern Africa contribute to the AI signal at the south of the tropical rain band (Prospero et al., 2002). We only analyzed and inter-preted the variability in the frequency of daily AI>1 at the north of the summer tropical rain band. The following data were used:

– Level 3 TOMS data of the period 1979–2001. TOMS data for the period 2002–2005 were not used due to cal-ibration problems (http://disc.sci.gsfc.nasa.gov/guides/ legacy-guides/tomsl3_dataset.gd.shtml).

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research/product/rowanomaly-background.php), the affected data are not included in the Level 3 data sets (http://disc.sci.gsfc.nasa.gov/Aura/data-holdings/OMI/ index.shtml#info).

Level 3 daily AI data of TOMS and OMI of summer (Au-gust) were downloaded from the Giovanni online data system of the NASA Goddard Earth Sciences Data and Information Services Center (GES DISC) (http://disc.sci.gsfc.nasa.gov/). The consistency of the TOMS and OMI AI data set has al-ready been shown (Li et al., 2009). Consistency between TOMS, OMI and our in situ dust measurements is analyzed in Sect. S3 of the Supplement (including Figs. S4 and S5). 2.3 Meteorological reanalysis data

We used gridded meteorological National Center for Envi-ronmental Prediction/National Center for Atmospheric Re-search (NCEP/NCAR) re-analysis data (Kalnay et al., 1996) to study the relationship between dust variability and large-scale meteorological processes in summer (August). This analysis included geopotential heights, winds and rains used in Eq. (1) (shown below) and Figs. 2, 4 and 9.

2.4 Summer dust season

At Izaña, the summer dust season (impacts of the SAL) typ-ically starts in the second half of July and ends at the begin-ning of September (Sect. S2 of the Supplement). The max-imum frequency of dust events occurs in August (52 % of the August days as average; Fig. 1). This month is of high interest given that (i) the ITCZ is shifted to the north and consequently (ii) the SAL is exported at the northernmost lat-itude (as evidence of the highest frequency of dust impacts at Izaña; Tsamalis et al., 2013) and the maximum rainfall oc-curs in tropical North Africa (Nicholson, 2009). For this rea-son, we used the August dust averages for studying summer long-term dust evolution in the boreal subtropics (Fig. 1a). The study of the central month (August) of the summer dust season (excluding July and September) allows one to charac-terize long-term evolution in terms of intensity of dust export, avoiding the variability that could be linked to (i) shifts in the beginning (July) or end (September) dates of the dust season or (ii) variability in the location of the ITCZ from July to September. Our data analysis shows that the July to Septem-ber dust average is dominated by the dust events occurring in August (Fig. S3 of the Supplement). In August 1987–2014, daily dust data were available during 761 days, i.e., a data availability of 94 % (excluding the no-measurement period 11 October 1999–13 February 2002). In this study we an-alyze 1987–2014 time series of in situ dust concentration at Izaña (determined by chemical methods) averaged on all (dust and no-dust) days of August (shown in Fig. 3a and ana-lyzed below). We refer to August as summer. Results are pre-sented in Sect. 4; additional analysis is prepre-sented in Sect. S3 of the Supplement.

3 North African summer meteorological scenario The meteorological scenario throughout western North Africa is influenced by the high pressures typical of the sub-tropical deserts and the so-called western African monsoon (Lafore et al., 2010). Additionally, the formation of the sum-mer Saharan heat low (Lavaysse et al., 2009) in the central western Sahara also has implications for meteorological pro-cesses, not only related to the development of the wet west-ern African monsoon season in tropical North Africa (Lafore et al., 2010), but also to mobilization, upward transport and export of dust to the North Atlantic at subtropical latitudes (Jones et al., 2003; Flamant et al., 2007; Knippertz and Todd, 2010).

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this dry dusty air mass over the North Atlantic free tropo-sphere resulting in the previously described SAL (Prospero and Carlson, 1972; Tsamalis et al., 2013). Off the coast of North Africa, the summer SAL is found at altitudes between 1 and 5 km (Karyampudi et al., 1999; Immler and Schrems, 2003; Tsamalis et al., 2013; Andrey et al., 2014) due to the westward dust export occurs above the so-called “Atlantic inflow”, a layer of cool and stable sea-breeze-like inflow present along the subtropical North African coast (Lafore et al., 2010).

This brief description illustrates how the presence of the dusty SAL over the North Atlantic is the net result of a set of complex and coupled processes that occur in a wide range of scales and that may also involve (i) feedback mechanisms (e.g., radiative, cloud and rain processes triggered by dust; Lau et al., 2009), (ii) interconnections between processes (e.g., influence of the AEJ–convection–monsoon connec-tions on dust described by Hosseinpour and Wilcox, 2014), (iii) variability in dust emissions due to meteorologically driven variability in soil features (Prospero and Lamb, 2003) and (iv) dust microphysical processes (e.g., size-dependent deposition and cloud and radiation interactions; Mahowald et al., 2014).

4 Results and discussion 4.1 The North African dipole

We aim to find a simple conceptual model for linking long-term variability in Saharan dust export with variability in the large-scale meteorology in western North Africa. Because it resembles a dipole, we will refer to the summer meteorolog-ical scenario of North Africa – characterized by high pres-sure in the subtropical Sahara (27–32◦N over Algeria; Font-Tullot, 1950; UK Meteorological Office, 1962) and low pres-sure in tropical North Africa (<15◦N) – as the North African dipole (NAFD). The NAFD is illustrated in the height of the 850 hPa summer average geopotential in Fig. 2a. The in-tensity of this dipole can be measured as the difference of the anomalies of the geopotential height over the subtrop-ics and that over the tropsubtrop-ics in North Africa. Because sum-mer dust export to the Atlantic occurs between 1 and 5 km altitude (Prospero and Carlson, 1972; Immler and Schrems, 2003; Tsamalis et al., 2013), with a frequent maximum dust load between 2 and 3 km (Tesche et al., 2009; Cuevas et al., 2015), we paid special attention to the 700 hPa standard level (Nicholson, 2013) at 5–8◦W longitude (i.e., close to the At-lantic coast). Thus, in this study we measured the intensity of the NAFD as the difference of the anomalies of the 700 hPa geopotential height averaged averaged over central Morocco (30–32◦N, 5–7◦W) and that over the Bamako region in Mali (10–13◦N, 6–8◦W) by Eq. (1). Other parameterizations of the NAFD are plausible depending on the study subject. We calculated the NAFD intensity (NAFDI) with Eq. (1) using

the average values of the 700 hPa geopotential heights in ev-ery month of August (31 days on average) from 1948 to 2014 obtained from the NCEP/NCAR re-analysis (Kalnay et al., 1996):

NAFDI= 1

10 8 y

Mo−< 8>Mo)−(8 y

Ba−< 8>Ba

, (1) where

8yMo is the mean geopotential height at 700 hPa aver-aged in the central Morocco region (30–32◦N, 5–7◦W) in August of year “y”.

< 8>Mo is the mean geopotential height at 700 hPa averaged in the central Morocco region (30–32◦N, 5– 7◦W) averaged in August months from 1948 to 2014. 8yBais the mean geopotential height at 700 hPa averaged

in the Bamako region (10–13◦N, 6–8◦W) in August of year “y”.

< 8>Bais the mean geopotential height at 700 hPa av-eraged in the Bamako region (10–13◦N, 6–8◦W) aver-aged in all August months from 1948 to 2014.

101 is a scale factor.

The NAFDI (Eq. 1) is a measure of the interannual vari-ability of the dipole intensity and, because of its relationship with the geopotential gradient, it is related to the intensity of the geostrophic North African outflow.

Figure 3a shows the time series of the summertime NAFDI values from 1987 to 2014, when it showed values between

−3.19 and +2.29. In order to assess how large-scale me-teorology changes with NAFDI values, we averaged some meteorological fields during high NAFDI summers and low NAFDI summers (Fig. 4a and c–e). The low NAFDI group includes the summers with the four lowest NAFDI val-ues in the period 1987–2014: 1997 (NAFDI = −3.19), 1987 (−2.79), 1996 (−2.04) and 2006 (−1.54). The high NAFDI group includes the summers with the four highest NAFDI values in the period 1987–2014: 2012 (+2.29), 2008 (+1.01), 2000 (+0.83) and 1988 (NAFDI= +0.68). Only summers for which satellite AI data are available were con-sidered (i.e., the 1993–1995 and 2002–2004 periods were not included in the selection). The spatial distribution of dust was assessed by determining the major dust activity frequency (MDAF) metric: the number of days with AI values>1 di-vided by the total number of days with available AI data in % (Fig. 4b).

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Figure 2. Height of the 850 hPa geopotential over North Africa averaged in (a) all summers (August) from 1987 to 2014 and in summers (August) with (b) low NAFDI values (1987, 1996, 1997 and 2006 have−2.79,−2.04,−3.19 and−1.54, respectively) and (c) high NAFDI values (1988, 2000, 2008 and 2012 have+0.68,+0.83,+1.01 and+2.29, respectively).

NAFDI summers (Fig. 4a1) than during high NAFDI sum-mers (Fig. 4a2).

4.2 Long-term variability of Saharan dust export At Izaña we observe a strong interannual variability in dust concentrations (Fig. 3a). In low dust years – 1987, 1997, 2006 and 2007 – mean concentrations were within the range 17–30 µg m−3; in high dust years – 1988, 2008, 2010 and 2012 – the range was 100–140 µg m−3. We asso-ciate this variability with the spatial variability of meteoro-logical conditions over North Africa, specifically with the NAFD (Fig. 4). The high value of the Pearson correlation co-efficient (r) of mean summer dustTat Izaña with the NAFDI

from 1987 to 2014 (r=0.72, Fig. 3a) indicates that the dust export is highly sensitive to the dipole intensity (Fig. 5a).

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repre-Figure 3. Long-term evolution (1987–2014) of summer dust and meteorology. Summer mean values of dustTconcentrations at Izaña (black dot, (a–d), MEI (green line, a), NAFDI (red triangle, a), zonal wind at 925 mb in the subtropical Saharan Stripe (25–28◦N, 7◦W–2◦E, b), zonal wind at 700 mb averaged in the subtropical Saharan Stripe to Tenerife corridor (25–28◦N, 16◦W–2◦E, c) and the Wet Sahel Portion (blue dot) from 1987 to 2014. Green and red arrows highlight moderate and intense ENSO and La Niña summers, respectively (http://www.cpc.ncep.noaa.gov).

sented by the NAFD (Fig. 2b and c). The subtropical Sa-haran Stripe region, which extends from central Algeria to the western Sahara between 24 and 30◦N, includes impor-tant dust sources (Prospero et al., 2002; Schepanski et al., 2009; Rodríguez et al., 2011; Ginoux et al., 2012) that clearly exhibit a greater MDAF during summers with high NAFDI (Fig. 4b2). Long-term (1987–2014) summer mean values of NAFDI and of dustTat Izaña are highly correlated with the zonal wind in the subtropical Saharan Stripe (r=0.6 to 0.9, Fig. 6a, where negative correlation indicates reinforcement of westward winds). These correlations reflect the net result of a wide range of dust-related processes (emission, vertical transport, advection to the Atlantic and size-dependent de-position during transport). These results are consistent with

those of the back-trajectory analysis (Fig. S6 of the Supple-ment).

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summer-to-Figure 4. North African dipole and spatial distribution of dust and meteorological fields averaged in low and high NAFDI summers. The low NAFDI group includes the summers with the four lowest NAFDI values in the period 1987–2014 (1987, 1996, 1997 and 2006 are−2.79, −2.04,−3.19 and−1.54, respectively). The high NAFDI group includes the summers with the four highest NAFDI values in the period

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Figure 5. Scatterplot of dust versus NAFDI and MEI. Summer mean dustTat Izaña (1987–2014) versus NAFDI (a), MEI (c) and NAFDI + MEI·(−1) (d), and summer mean dust2.5-to-dustT ra-tio (b, 2002–2014) versus NAFDI. Different symbols are used for the summer mean data of 2002 (black triangle), 2010 (grey triangle) and 2012 (white filled symbol) for examining how some data may have different associations with NAFDI and MEI.

summer variability in zonal winds in the subtropical Saharan Stripe (Figs. 4c and 6a) and dust export at subtropical lati-tudes. Reinforcement of easterly winds during high NAFDI summers is also observed in the AEJ (Fig. 4d), which plays a role in the trans-Atlantic dust transport (Jones et al., 2003). 4.3 Long-term variability of dust size distribution Our dust record in two size fractions was used to assess long-term variability in dust size distribution. We found that the NAFDI is correlated with the interannual variability of dust size distribution. Our measurements show pronounced changes in the size distribution of dust particles that are apparently related to wind interannual variability driven by the NAFDI (Fig. 5b). Dust tends to be coarser during high NAFDI years than during low NAFDI years. Observe how the dust2.5to dustTratio tends to decrease with the NAFDI increase:∼30 % in summers with NAFDI<0 and down to

∼20 % in summers with NAFDI >2 (Fig. 5b). The high amount of coarse (>2.5 µm) dust during high NAFDI sum-mers may be linked to the activation of dust sources closer to the Atlantic coast and/or faster atmospheric transport due to higher wind speeds. Both processes will reduce the loss rate of larger-size particles due to gravitational deposition during transport (Ryder et al., 2013).

4.4 Connection of NAFD to climate variability

In this section we assess if the NAFDI could be used for link-ing long-term export of Saharan dust with climate variability during the last decades. Here we present some associations between NAFDI, tropical rains and ENSO that will require future investigations.

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Figure 7. Scatterplot of summer dust activity in the subtropical North Atlantic versus the NAFDI. MDAF in the subtropical North Atlantic (SNA) versus the NAFDI (a) and versus dustTat Izaña (b). Measurements of the TOMS (red circle) and OMI (blue dot) satellite-borne sensors were used. TheR2coefficient of the linear fitting is included.

Figure 8. Summer mean values of NAFDI (red triangle, a), dustTat Izaña (black dot, b) and zonal wind at 925 mb in the subtropical Saharan Stripe (25–28◦N, 7◦W–2◦E, b). Period of plentiful rains and severe drought in the Sahel are highlighted according to Lucio et al. (2012).

in the Caribbean found by Prospero and Lamb (2003). The long-term (1987–2014) correlation of dustTat Izaña with the Wet Sahel Portion (r=0.74, Fig. 3d) suggests that variabil-ity in the Saharan dust export in the subtropical and monsoon tropical rains has been influenced by a common meteorologi-cal/climatic mechanism. Observe how high dustTsummers at Izaña have been associated with the high Wet Sahel Portion in the last three decades (e.g., 1988, 1999, 2010, 2012 and 2013, dustT=75–140 µg m−3 and Wet Sahel Portion=7– 15 %; Fig. 3d) and vice versa (e.g., 1996, 1997, 2006, 2009, 2011, 2014, dustT=17–45 µg m−3 and Wet Sahel Portion

=0.8–4.5 %; Fig. 3b). On a shorter timescale (days to weeks), this connection of dust export and monsoon rains was also observed by Wilcox et al. (2010), who found that the tropical rain band shifted northward by 1 to 4◦latitude during west-ward dust outbreak events accompanied by an acceleration of winds on the northern edge of the AEJ.

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Figure 9. Mean winds at 925 hPa (≈800 m a.s.l.) in the 1957–1967 (plentiful rains in the Sahel) and 1980–1990 (severe Sahelian drought) decades.

surface air temperature and total cloudiness fraction of the sky over the tropical Pacific Ocean – is moderately corre-lated with the NAFDI (r= −0.50) and with dustT at Izaña (−0.59) (Fig. 1a) (Table S2 of the Supplement). Because variability in the NAFDI is connected to wind at the north of the ITCZ (Fig. 6a), these correlations suggest that MEI may be teleconnected to winds over the subtropical Sahara, and this would have implications for Saharan dust export. In five of the eight intense ENSO years recorded from 1987 to 2014 (green arrows at the top of Fig. 3), dustTconcentrations at Izaña were low (1987, 1997, 2006, 2009 and 2014, 17– 32 µg m−3; Fig. 3a) coupled with rather low zonal winds at 925 and 700 mb along the subtropical Saharan Stripe (Fig. 3b and c), whereas in the other three intense ENSO years, dustT concentrations were moderate (1991, 1993 and 2002, 47– 61 µg m−3; Fig. 3a). In the 1987–2014 time series, we can observe that many of the peak dustTsummers are associated with correlated increases in NAFDI and MEI · (−1) (e.g., 1988, 1998 and 2008); however, we also observe some peak dustT summers associated with MEI peaks but rather low NAFDI values (e.g., 2002 and 2010, Fig. 3a) and vice versa, i.e., peak dustT summers associated with NAFDI peaks but rather low MEI values (e.g., 2012). This suggests that NAFDI and MEI may be tracing the dependence of different pro-cesses involved in dust export on climate variability (e.g., regional variability in source activation, spatial distribution of dust or altitudinal and latitudinal shifts of the SAL). Ob-serve how long-term summer mean dustT at Izaña exhibits higher linearity with NAFDI + MEI ·(−1) (Fig. 5d,R2=

0.60) than with either the NAFDI (Fig. 5a, R2=0.52) or MEI (Fig. 5c, R2=0.34). The 1987–2014 summer mean dustT at Izaña exhibited a higher correlation with NAFDI + MEI · (−1) (r=0.77) than with NAFDI (r=0.72) or MEI · (−1) (r=0.50). Teleconnections of dust with sev-eral large-scale systems were also observed by Doherty et al. (2008), who found that trans-Atlantic transport of dust was teleconnected to displacement of both the Azores and Hawai-ian anticyclones. Deficits in the North African tropical rains

have also been linked to ENSO (including summer Palmer, 1986; Bhatt, 1989; Janicot et al., 1996; Rowell, 2001), con-sistent with the correlation found between the NAFDI and precipitation rates over tropical North Africa (Fig. 6c) and with the low Wet Sahel Portions we observe in low NAFDI and MEI·(−1) summers (Fig. 3a and d). Interannual vari-ability in dust transport in subtropical Asia (Abish and Mo-hanakumar, 2013) and dust mobilization in sources affected by land use and ephemeral lakes (Ginoux et al., 2012) have also been linked to ENSO.

The increase in the concentrations of dust transported to the tropical North Atlantic – at Barbados – since the mid 1970s has been linked to Sahelian droughts (Prospero and Lamb, 2003). Figure 8a shows the summer NAFDI val-ues from 1950 to 2014. Valval-ues of NAFDI were persistently higher prior to the onset of the Sahelian drought – from the 1950s to the mid 1960s – than since the mid 1970s, with the lowest values observed during the most severe part of the drought – from 1980 to 1990 (Fig. 8a). Similarly, sum-mer mean values of zonal wind at 925 mb in the subtropi-cal Saharan Stripe were persistently higher prior to the Sahe-lian drought (Fig. 8b). This suggests that the meteorological change that occurred in the mid 1970s did not only occur in the Sahel, but also in the subtropical Sahara. Particularly, the high wind speeds in the subtropical Saharan Stripe between the mid 1950s and the mid 1960s (Fig. 9a) – e.g., compared to the 1980–1990 period (Fig. 9b) – may have enhanced dust mobilization in the central Sahara (north of the ITCZ, includ-ing the subtropical Saharan Stripe). Further studies should address what the implications have been for dust transport paths and impacts over the North Atlantic of such meteoro-logical changes.

5 Conclusions

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Tener-ife) and satellite and meteorological reanalysis data shows that summer Saharan dust export is highly dependent on the variability of the large-scale meteorology in North Africa, which is characterized by a high over the subtropical Sahara and a low over the tropics linked to the monsoon (at 850 hPa and aloft). We referred to this high–low dipole-like pattern as the North African dipole (NAFD) and, in this study, we parameterized its variability in terms of the NAFD intensity (NAFDI): the difference of geopotential height anomalies av-eraged over the subtropics (30–32◦N, Morocco) and the trop-ics (10–13◦N, Bamako region) close to the Atlantic coast (at 5–8◦W longitude). Because summer dust export off the coast of North Africa tends to occur between 1 and 5 km a.s.l., we determined the NAFDI at the 700 hPa standard level. Other parameterizations of the NAFD are plausible, depending on the study subject.

We observe significant summer-to-summer variability in the NAFDI, which is associated with shifts in the Saharan high that have implications for winds over the Sahara and for dust export in the North African outflow. Increases in the NAFDI values (i) result in higher wind speeds at the north of the Inter-Tropical Convergence Zone that are associated with enhanced dust export over the subtropical North Atlantic, (ii) influence the size distribution of exported dust particles (in-creasing the load of coarse dust) and (iii) are associated with enhanced rain in the tropical and northern shifts of the trop-ical rain band that may affect the southern Sahel. Variability in the NAFDI is also connected with spatial distribution of dust over the North Atlantic; high NAFDI summers are asso-ciated with major winds and dust export in the subtropics and minor dust presence in the tropics (linked to rainfall scaveng-ing), and vice versa.

We found connections of the NAFDI and dust at Izaña with climate variability. El Niño periods (e.g., 1987, 1997, 2006, 2009 and 2014) are generally associated with moder-ate to low summer mean values of the NAFDI, wind speed at the north of the ITCZ and dust at Izaña, and vice versa during La Niña summers (e.g., 1988, 1998, 1999 and 2010). The 1987–2014 summer mean dust records at Izaña showed a higher correlation with NAFDI+MEI ·(−1) (r=0.77) than with either NAFDI (r=0.72) or MEI·(−1) (r=0.50). These correlations show the need for understanding the pro-cesses that link dust to climate variability in the subtropics and tropics.

Further studies are necessary to understand how the vari-ability of the summer NAFDI since 1950 to the present day – associated with high wind speeds over subtropical Saha-ran dust sources prior to the Sahelian drought and low wind speeds over the subtropical Sahara during the severe part of the drought – may have influenced the multi-decadal evolu-tion of the dust export paths.

The Supplement related to this article is available online at doi:10.5194/acp-15-7471-2015-supplement.

Acknowledgements. The Izaña GAW program is funded by AEMET and by the Minister of Economy and Competitiveness of Spain (POLLINDUST, CGL2011-26259). We gratefully ac-knowledge the cooperation of the NOAA/ESRL Physical Sciences Division, the NASA Goddard Earth Science Data and Informa-tion Services Center and the NOAA Air Resources Laboratory. J. M. Prospero’s research is supported by NSF grant AGS-0962256. M. I. García holds a grant from the Canarian Agency for Research, Innovation and Information Society and the European Social Fund. We thank our colleague Celia Milford for the comments and suggestions that improved the original manuscript.

Edited by: U. Pöschl

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